At stake is RTD's grand vision of rail lines shooting like spokes in a wheel from Union Station in Lower Downtown.

The transit agency wants to decouple the rail and highway components of its $4.5 million FasTracks transportation plan.

Up to now, state officials have insisted that rail, paid for by RTD, be developed at the same time as highways, paid for by the state. The problem is that lawmakers have had to slash $700 million from the state's budget this year.

In a recent letter to Colorado Department of Transportation chief Tom Norton, RTD general manager Cal Marsella sought to split the rail and highway portions of FasTracks.

"CDOT is not required to obligate or expend any highway dollars in order for RTD to proceed with construction of the rail lines for FasTracks," Marsella said.

Marsella's letter comes as RTD prepares this week to open its newest line, the 1.8-mile Central Platte Valley spur between Union Station and the Auraria Higher Education Center.

As they celebrate the $46 million line that will take transit riders to the Pepsi Center and Mile High stadium, RTD officials are refining their year-old strategy aimed at having rail lines run throughout the metro area from Union Station.

FasTracks would add rail lines to Lakewood-Golden, Arvada, Boulder, Thornton-Northglenn, Denver International Airport and Aurora's Interstate 225 corridor.

To pay for the new transit lines, RTD would like to raise its sales-tax portion in the seven-county metro area to a full penny on each dollar's purchase, from the current 0.6 cents per dollar.

Some of the FasTracks routes, notably the U.S. 36 and I-225 corridors, may need hundreds of millions of dollars in highway improvements to accompany the new rail lines, according to Colorado transportation officials.

Yet a shrinking transportation budget has made it impossible for the administration of Gov. Bill Owens to consider committing funds to the highway side of FasTracks.

Until recently, RTD officials acknowledged that their preference was to collaborate with CDOT in building transportation corridors throughout the metro area that blend highway expansion with rail transit where appropriate.

The model for such "multimodal" projects is T-REX, the $1.67 billion light-rail and highway expansion project being constructed along I-25 and I-225.

Its 19 miles of southeast corridor light rail and 17 miles of expanded interstate highway are due to open in late summer 2006.

If a tax increase isn't approved for FasTracks, RTD will have only enough money to build one more rail line in the metro area over the next 20 years, said Liz Rao, the transit district's director of planning and development.

To take FasTracks to voters, RTD needs the approval of the Colorado General Assembly. A bill to give RTD that right has passed the Senate, and a House committee will take up the measure this week.

But as they await legislative action on the bill, RTD officials are positioning themselves to wriggle out of the state's budget dilemma.

"A lot of decisions have to be made about whether or not we have relief for congestion in the metro area and what is the future of transit," Rao said. "We do not require any investment of CDOT to make our project happen. If they don't have enough money, we still can do the rail corridors."

Yet CDOT's Norton says it is a mistake to divide the transit and highway investments.

"My contention is that it's not about the mode of transportation - it's not rail vs. roadway - but rather is there a system that works together to most economically move people and goods?" he said.

Colorado Forum director Gail Klapper said she has been a champion of RTD's proposed "full build-out" throughout the metro area.

But opposition from the Owens administration to such a comprehensive transit plan may make it more practical to "prioritize" the next rail line to be built, said Klapper, whose group includes some of the state's leading chief executives.

"I'd love to see a full build-out," Klapper said. "This community needs to have a strong transit component. But sometimes it's the art of the possible. I'm looking for a place we can begin."

Up to now, RTD officials have insisted that rail lines should be built throughout the area and that no one route should take priority over another.

Boulder Mayor Will Toor said he supports such a regional approach.